1. Field of the Invention
The present invention relates to a damping force control type hydraulic shock absorber attached to a suspension system of a vehicle, for example, an automobile.
2. Description of the Background Art
Hydraulic shock absorbers attached to suspension systems of automobiles or other vehicles include damping force control type hydraulic shock absorbers which are designed so that the level of damping force can be properly controlled in accordance with the road surface conditions, vehicle running conditions, etc., with a view to improving the ride quality and the steering stability.
In general, a damping force control type hydraulic shock absorber is arranged as follows. A piston with a piston rod connected thereto is slidably fitted in a cylinder having a hydraulic fluid sealed therein. The hydraulic fluid in the cylinder is induced to flow by the sliding movement of the piston caused by the extension and contraction of the piston rod, and the flow of the hydraulic fluid is controlled by using orifices, disk valves, etc., thereby generating damping force. Further, the damping force is controlled by varying the hydraulic fluid passage area with a damping force control mechanism. The damping force control mechanism is generally built in the piston assembly in the cylinder. The damping force control mechanism has a valve body which can be actuated through a control rod extending through the piston rod. Thus, damping force control is effected by externally moving the valve body of the damping force control mechanism through the control rod.
There is another type of damping force control type hydraulic shock absorber in which a hydraulic fluid passage is provided outside the cylinder, and a damping force control mechanism is provided on the side of the cylinder, thereby reducing the size of the piston assembly, and thus enabling the stroke of the piston rod to be increased, as disclosed, for example, in Japanese Patent Application Unexamined Publication (KOKAI) No. 4-312227. In this type of damping force control type hydraulic shock absorber, the damping force control mechanism is disposed on the side of the cylinder such that the valve body of the mechanism moves along an axis perpendicularly intersecting the center axis of the piston rod, thereby preventing the movement of the valve body from being affected by the acceleration of the cylinder unit caused by vibration of an unsprung member of the suspension system.
However, the above-described conventional damping force control type hydraulic shock absorber, in which the damping force control mechanism is provided on the side of the cylinder, suffers from the problem that, when the overall length of the valve body is long, the amount to which the damping force control mechanism projects from the side of the cylinder becomes large, thus unfavorably limiting the space for mounting the shock absorber on the vehicle.
The overall length of the valve body is particularly long in a damping force control mechanism in which an extension-side passage and a compression-side passage, which are separately provided, are opened and closed with a single spool valve (valve body) in order to enable setting of a combination of damping force characteristics which are different for the extension and compression sides in terms of the magnitude of damping force (e.g. a combination of "hard" damping force characteristics for the extension side and "soft" damping force characteristics for the compression side, or vice versa). Therefore, the amount to which the damping force control mechanism projects becomes particularly large; this gives rise to the problem of interference between the damping force control mechanism and the vehicle body.